Fix light client call_executor execution_proof_is_generated_and_checked test

[jordy25519]

Migrated from: cennznet/plug-blockchain#62 (comment)

[cowboy-bebug]

On master branch:

failures:

---- tests::voter_catches_up_to_latest_round_when_behind stdout ----
thread 'tests::voter_catches_up_to_latest_round_when_behind' panicked at 'assertion failed: set_state.read().last_completed_round().number >= 4', core/finality-grandpa/src/tests.rs:1501:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.


failures:
    tests::voter_catches_up_to_latest_round_when_behind

test result: FAILED. 58 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out

error: test failed, to rerun pass '-p substrate-finality-grandpa --lib'

cargo test -p substrate-finality-grandpa --lib passes however

[cowboy-bebug]

The cause of error seems to be this:

panic_handler::set("TEST", "1.2.3");

It's a library from substrate that wraps around std::panic::set_hook() (src).
^ I guess one way to fix this bug is to make the test not panic even if panic_handler is set (which is what it's actually testing)

[cowboy-bebug]

This test also fails on substrate/master with the same error log:

cargo test -p substrate-client light::call_executor::tests::execution_proof_is_generated_and_checked

Error:
Thread 'light::call_executor::tests::execution_proof_is_generated_and_checked' panicked at 'Externalities not allowed to fail within runtime: "Trie lookup error: Database missing expected key: 0xb3fa3eff4d4d5d287ebc1be63f1d0236b2c21aaea7d0c20fe1e4ca597a0705b3"', src/libcore/result.rs:1165

Our error just has a different key: 0xd8a1f856f5faa67db595950a0b5a965e03b3499aa48c3a05d1f8e948f81a1854

Upon closer examination, the issue is more related to not being able to write to the trie database. Can track this comment: https://matrix.to/#/!HzySYSaIhtyWrwiwEV:matrix.org/$1571969957149258GIffF:matrix.org?via=matrix.org&via=matrix.parity.io

[jordy25519]

Seems to be resolved on latest develop